You're given a list of customer arrival and service times. Calculate the average waiting time for all customers, assuming they are served in the order they arrive.

Given a sorted array, find the first and last position of a given target value. If the target is not found, return [-1, -1].

Determine if every row and every column in a given square matrix contains all the numbers from 1 to n, where n is the size of the matrix. Effectively, check if it's a valid Sudoku (without subgrids).